1. Field of the Invention
The present invention relates to a process and system for gasifying biomass or other carbonaceous feedstocks in an indirectly heated gasifier and a method for the elimination of condensable organic materials (tars) from the resulting product gas with an integrated tar removal step. More specifically this integral tar removal step utilizes the circulating heat carrier to crack the organics and produce additional product gas. As a benefit of the above process and because the heat carrier circulates through alternating steam and oxidizing zones in the process, deactivation of the cracking reactions is eliminated.
2. Description of the Related Art
The related art involves a biomass gasification system and method as described in U.S. Pat. No. 6,808,543 (Paisley), the contents of which are incorporated herein fully by reference (also referred to as the FERCO system).
The '543 patent proves a gasification method involving a circulating fluidized bed (CFB) gasifier system wherein sand is used as a heat transfer medium. A first aspect of the system provides a method for reducing ash agglomeration in a parallel entrainment fluidized bed gasifier/combustion wherein Magnesium Oxide (MgO) is added into the thermal transfer material to alter a eutectic of the resultant ash and thereby raise a melting point of such ash to minimize agglomeration with the sand. In this way the '543 patent teaches the need to minimize sand+ash agglomeration through an increase of the ash melting point by adding MgO. Paisley '543 distinguishes itself from prior systems adding CaO and Al2O3 in attempts to reduce agglomeration by diluting ash.
The related art also involves a method for hot gas conditioning as described in U.S. Pat. No. 5,494,653 (Paisley), the contents of which are also incorporated herein fully by references.
The '653 patent discusses the production of a feed gas for hydrogen synthesis using gasification in a fluidized bed reactor (FBR), recirculating fluidized bed reactor (CFB), or in a fixed bed reactor (FB), and requires the use of a catalyst to adjust the hydrogen to carbon monoxide ratio in a water gas shift reaction,CO+H2O=CO2+H2  Equation 1in such a way as to not promote the formation of carbon, an undesired byproduct. Patent '653 teaches that alumina is a preferred catalyst and that conventional catalyst systems and methods for such reactions require the use of noble metals such as nickel, molybdenum, and the like, or of alkali materials such as potassium, sodium, and the like for catalysts. Further, conventional catalyst systems and methods do not suppress carbon to the extent desired in the '653 patent. Typical of these and other gas production operations are the following U.S. Pat. No. 233,861 to Jerzmanowski; U.S. Pat. No. 1,295,825 to Ellis; U.S. Pat. No. 1,875,923 to Harrison; U.S. Pat. No. 1,903,845 to Wilcox; U.S. Pat. No. 1,977,684 to Lucke; U.S. Pat. No. 1,992,909 to Davis; U.S. Pat. No. 2,405,395 to Bahlke et al; U.S. Pat. Nos. 2,546,606; 3,922,337 to Campbell et al; U.S. Pat. No. 4,726,913 to Brophy et al; U.S. Pat. No. 4,888,131 to Goetsch et al; U.S. Pat. No. 5,143,647 to Say et al; and British patent GB 461,402 (Feb. 16, 1937).
The '653 patent teaches a method of crackling and shifting a synthesis gas by providing a catalyst consisting essentially of alumina in an out-of-the-re-circulating-path chamber; and contacting the alumina catalyst with a substantially oxygen free synthesis gas of methane and/or higher molecular weight hydrocarbons; and water vapor at a temperature of about 530° C. to about 980° C., whereby methane and higher hydrocarbons are cracked according to the reaction,CxH2y+xH2O=xCO+(1+y+x)H2  Equation 2
and shifted by the reaction in Equation 1 (water gas shift reaction), whereby carbon formation is minimized at temperatures below 980° C.
As a consequence, the '653 patent teaches the use of alumina as a gas conditioning catalyst that can shift the so called water gas shift reaction to enable a ratio of H2:CO of 2:1.
While the mechanical system in the '653 patent is generally similar to that in the '543 patent, a difference exists for the above-noted out-of-cycle (ex situ) lower temperature reaction chamber (maximum 815° C./1500° F.) that receives an output synthesis gas from the gasification module and contains the alumina catalyst for down-stream reaction. As a consequence, the '653 patent differs from that of the '543 system by providing both (i) an alumina (Al2O3) catalyst and (ii) providing that catalyst only in an ex situ chamber of low temperature that results in rapid downstream deactivation of the cracking reactions due to the deposition of carbon on the catalyst surface.
These related art references fail to appreciate a number of aspects noted in the present invention. These aspects include the use of a re-circulating heat transfer medium containing a catalyst, the removal of hydrocarbons (condensables) at high temperatures above about 370° C., and the removal of condensables in an in situ process (continuous removal with recycle).
As a consequence, a number of aspects of the present invention are not appreciated by the prior art. These include the ability of the present invention to recover heat to a significantly lower temperature following an in situ condensable removal step (due to the removal of tars from the gas at high temperature).
Similarly, the related art fails to appreciate the benefit of a separate gas-conditioning module that is heated in situ at high system temperatures (1000° C./1800° F.) by a thermal transfer medium heated in a combustor module and recycles for continuous use, thereby increasing the efficiency of all reactions due to higher temperature use.
Furthermore, the related art fails to appreciate the ability for increased hydrogen (H2) recovery by including an in situ gas-conditioning module that operates at system high temperatures.
Finally, the related art in all cases fails to appreciate the use of steam in situ as a reactive to impact the water gas shift reaction in gas conditioning and thereby minimize the requirement for removal of condensables in a separate (ex situ) and costly downstream process
Accordingly, there is a need for an improved process that enables removal of condensable products in situ and which therefore enables, but does not require, an improved energy recovery from product gas.